Download 5 Alkenes and Alkynes GOB Structures

12.5 Alkenes and Alkynes
H
C
C
H
Ethyne, commonly called
acetylene, is used in
welding, in which it reacts
with oxygen to produce
flames with temperatures
above 3300 °C.
Learning Goal Identify structural formulas as alkenes,
cycloalkenes, and alkynes, and write their IUPAC names.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Alkenes and Alkynes
Alkenes and alkynes are families of hydrocarbons that
• contain double and triple bonds, respectively.
• are called unsaturated hydrocarbons because they do not
contain the maximum number of hydrogen atoms.
• react with hydrogen gas to increase the number of
hydrogen atoms and become alkanes.
Remember, carbon atoms always form four covalent bonds.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Identifying Alkenes
Alkenes contain one or more
carbon–carbon double bonds.
In ethene (C2H4), two carbon
atoms are connected by a
double bond.
Each carbon atom in the
double bond is attached to two
hydrogen atoms and has a
trigonal planar arrangement
with bond angles of 120°.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Ethene (C2H4)
Ethene (C2H4), more
commonly called ethylene,
• is an important plant
hormone involved in
promoting the ripening of
fruits such as bananas.
• accelerates the breakdown
of cellulose in plants, which
causes flowers to wilt and
leaves to fall from trees.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Identifying Alkynes
Alkynes contain one or more
carbon–carbon triple bonds.
In ethyne (C2H2),
• two carbon atoms are
connected by a triple bond.
• each carbon is also bonded to
one H atom.
Each carbon atom in the triple
bond has a linear arrangement
with bond angles of 180°.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Study Check
Identify each of the following compounds as an alkene or alkyne:
A.
B.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Study Check
Identify each of the following compounds as an alkene or alkyne:
A.
alkene; contains a double bond
B.
alkyne; contains a triple bond
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Naming Alkanes, Alkenes, and Alkynes
The IUPAC names for alkenes and alkynes
• are similar to those of alkanes.
• use the alkane name with the same number of
carbon atoms, replacing the ane ending with ene.
Cyclic alkenes are named as cycloalkenes.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Guide to Naming Alkenes and Alkynes
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Study Check
Write IUPAC names for each of the following:
A.
B.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Solution
STEP 1 Name the longest carbon chain that contains the
double or triple bond.
ANALYZE
Given
Need
A.
THE
PROBLEM
five-carbon chain,
double bond,
methyl group
IUPAC name
five carbon atoms—pentene
B.
ANALYZE
THE
PROBLEM
Given
six-carbon chain,
triple bond
Need
IUPAC name
six carbon atoms—hexyne
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Solution
STEP 2 Number the carbon chain, starting from the end
nearer the double or triple bond.
1
3
5
A.
2
2-pentene
4
5
B.
4
3
2
1
6
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
2-hexyne
© 2016 Pearson Education, Inc.
Solution
STEP 3 Give the location and name of each substituent
(alphabetical order) as a prefix to the alkeneor
alkyne name.
1
3
5
A.
4-methyl-2-pentene
2
4
5
B.
4
3
2
1
6
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
2-hexyne; there are no
substituents in this compound.
© 2016 Pearson Education, Inc.
Naming Cycloalkenes
Cycloalkenes have a double bond within a ring structure and
• are named by assigning the double bond to be between
carbon 1 and carbon 2 when a substituent is on the ring.
• do not need to include the numbers for the double bond.
3-methylcyclopentene (It is
understood that the double bond is
between carbon 1 and carbon 2.)
The carbon atoms in the ring are numbered to give the double
bond numbers 1 and 2 and the lowest possible number to any
substituents present.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Study Check
Name the following alkenes and alkynes:
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Solution
Name the following alkenes and alkynes:
1
cyclohexene
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
2
3
3,3-dimethylcyclopentene
© 2016 Pearson Education, Inc.
Chemistry Link to the Environment:
Fragrant Alkenes
The odors you associate
with lemons, oranges,
roses, and lavender come
from the volatile compounds
that are synthesized by the
plants that produce these
flowers and fruits.
Often these compounds
contain double or triple
bonds or ring structures;
they are unsaturated.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Chemistry Link to the Environment:
Fragrant Alkenes
In ancient times, these compounds were highly valued in
their pure forms.
• Limonene and myrcene give the characteristic odors and
flavors to lemons and bay leaves, respectively.
• Geraniol and citronellal give roses and lemongrass their
distinct aromas.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.